Frequently Asked Questions About Home Inspections

FAQs

Frequently Asked Questions

1.What is an Automatic Weather Station?

An Automatic Weather Station, also known as an AWS or AWOS or weather station, is a system that continuously records weather parameters. The main components of a weather station are the Data Logger and the weather sensors such as rainfall, wind speed, wind direction, ambient air temperature, relative humidity, solar radiation, barometric pressure etc. Additional components include telemetry or remote connection, typically by GSM, GPRS, UHF or VHF, GSM.

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2.Weather Station Siting

How and where to site a weather station is one of the most frequently asked questions. The main determinants of how & where to site a weather station and sensors are the intended application, and the surrounding environment of the desired location itself.

Microclimate Versus Meso-Scale Monitoring:

A weather station records the weather exactly at the point it is located. In some applications, we wish to infer that those readings are valid for a radius of 1 to 2 kilometres. This is called a ‘microclimate’ application. In other situations, we would like to infer that the readings are valid for up to 25 kilometres. This is called a ‘Meso-scale’ application.

In choosing a location for your weather station, you must first decide whether you wish to record either:

a) The weather specifically relevant to your location i.e. the weather experienced by your building or crop including the effects of your immediate local terrain (Microclimate monitoring).

or:

b) The weather of the general locality (say within a 25 km radius), but as far as possible unaffected by the local terrain (Meso-scale weather monitoring).

Microclimate siting

Requirements for microclimate monitoring (a) are less stringent, and data is usually used for comparison purposes with a similar crop / building in another location. The other primary uses for microclimate monitoring include where the specific effects of the surrounding terrain are what is required. This includes, building control situations, sewage treatment plants, feedlot animal comfort studies, and most horticultural uses.

The station should be located as close as possible to the area under investigation. For example, in crop trials, the preferred siting would be on one side of the crop. It is important to choose a site that is clear of trees or overhangs. Nearby trees should be no higher than one quarter of their distance away.

Meso-scale siting

Requirement (b) is more stringent, and to achieve optimum results, the WMO and IMD guidelines should be followed. This includes a 10-metre mast for wind speed and direction sensors and solar radiation sensors, with trees or other obstacles no higher than one tenth (preferably one thirtieth 1/30) of their distance away. The terrain should be flat and level.

These requirements may be difficult to achieve in practice, therefore some compromise is usually required.

Some of the main uses for a 10-metre mast are to monitor the meso scale effects of wind in industries such as feedlots, manufacturing and mining, in accordance with satisfying EPA licence conditions. This is particularly important when an industry produces odours, dust, or other airborne particles that may impact on the surrounding environment.

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3.What is Relative Humidity?

Relative Humidity is the most commonly used measurements of moisture content in the air. The key to understanding relative humidity is to understand that it is a measure of the ‘actual humidity’, relative to the maximum possible humidity at a given temperature. Let’s explain it a bit further. Consider the air to be like a big sponge that can soak up moisture. Also consider that this ‘sponge’ can hold more moisture as it gets hotter, and less moisture as it cools.

Then the Relative Humidity is the amount of water vapour (moisture) in the air compared to the maximum amount that the air could hold at a given temperature.

The relative humidity is expressed as a percentage, so the maximum is 100 %. The formula for relative humidity is:

Relative humidity % =Moisture in the air now / Maximum possible moisture air can hold at the current temperature (x100)

The diagram below illustrates how the relative humidity changes with temperature. In this example, a volume of air contains a specific, (fixed) amount of moisture. As the volume of air is heated, the relative humidity decreases. As the volume of air is cooled, the relative humidity increases.

In the above example, the actual moisture content stays constant; it is the potential of the air mass to hold moisture that is changing with the temperature. That is why an increase in temperature is reflected as a reduction in the relative humidity.

The diagram below shows lines of constant relative humidity. The horizontal axis represents air temperature in degrees Celsius (ºC) and moisture content in g/kg of air on the vertical axis. Note that the same relative humidity value can exist at different temperatures and different moisture contents (follow the up-sloping curves).

Applications:

Relative Humidity readings are used to determine human and animal comfort levels, predict the likelihood of certain crop diseases, and is used in certain manufacturing environments. The effectiveness of crop spraying is also determined by the relative humidity in conjunction with wind and other climatic variables.

Please refer to our article on “How to read a psychrometric chart” for information on determining relative humidity using wet bulb and dry bulb thermometers, as well as determining a wet bulb reading or Dew Point temperature from relative humidity and air temperature.

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4.Reading a Psychrometric Chart

Relative Humidity can be calculated with a wet bulb thermometer and a dry bulb thermometer, using a psychrometric chart as below. Wet-bulb temperature is measured using a thermometer, with the glass bulb or sensor tip wrapped in a wick, which is kept wet. The evaporation of water from the wet wick has a cooling effect on the thermometer and the rate of evaporation from the wet-bulb thermometer depends on the humidity of the air. Therefore, less humidity means more cooling, which means a lower wet bulb temperature, and a bigger difference between wet and dry bulb temperatures.

The difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity. This chart allows humidity to be calculated from wet and dry bulb (air temperature) readings. It also explains common atmospheric behaviour that we see daily. Break the chart for ease of explanation:

The vertical lines represent dry bulb temperatures.

The diagonal lines represent wet bulb temperatures.

Lines of constant humidity are also shown on the chart

Read the relative humidity by finding where the wet bulb temperature and the dry bulb temperature readings intersect. The nearest constant humidity line is the relative humidity for the given wet and dry bulb temperatures. In the example to the left (20°C dry and 10°C wet) gives 25% relative humidity.

You can also read the wet bulb temperature from the chart, when you know the air temperature and relative humidity, using the same principle of finding the intersection of the relevant lines.

For more information on Relative Humidity, see our Relative Humidity article.

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5.How to determine 'Dew Point Temperature'?

The dew point temperature, commonly referred to as the ‘Dew Point’, is a measure of the amount of moisture in the air.

Consider the air to be like a big sponge that can soak up moisture. Also consider that this ‘sponge’ can hold different levels of moisture at different temperatures. As the air gets warmer it is able to hold more moisture. Conversely, the air can hold less moisture as it cools.

The dew point is the temperature at which the air, if cooled with its current moisture level, can hold no more moisture. That is, it is ‘saturated’ (with moisture).

It also follows that cooling the air further will result in some form of precipitation – that is, fog, rain, dew, sleet, hail or snow.

The dew point temperature can be calculated from the air temperature and relative humidity.

Relative Humidity is another means of expressing the amount of moisture in the air. Please see our separate article on Relative Humidity for more information.

Common uses for Dew Point:

The dew point temperature can be a useful predictive tool in combination with other weather sensors, to predict precipitation and frosts. This has applications across several industries and is particularly useful for agriculture and aviation.

Pilots often use the Dew Point as it highlights the difference between the air temperature now and the air temperature at which fog is likely to form. The closer the two temperatures are, the higher the risk of fog, which is their concern.

Dew point is also used in spraying applications, where it can be used to determine the likelihood of heavy dew overnight, which can impact upon the spray procedures. If the dew point temperature is close to the temperature expected overnight, then there is an increased likelihood of dew or fog formation.

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6.Pollutant Drift and Inversion Layers

The movement of industrial airborne particles such as smoke, dust or odour, is often much more complex than wind directions and speeds indicate. In most cases it is desirable for the pollutant to be highly dispersed, thereby staying under allowable limits, and avoiding issues with your neighbours. The factors that are most likely to effect the dispersion of a pollutant are net wind speed and direction, the variability of wind speed and direction and inversion layers.

Having a weather station on site at industry is the best way to detect if an inversion layer is present, as well as record the wind speed and direction to determine the characteristics of dispersion.

Inversion layers

The air temperature is normally warmest at the earth's surface and steadily reduces as you go up into the atmosphere.

Hot air is less dense (lighter) than cold air. Therefore, normally, the warm air at the surface slowly rises taking any air pollutants with it. This helps disperse the pollutants so that they are undetectable to humans and not a nuissance to immediate neighbours.

However, if a still or stable layer of colder air sits above warmer air, it forms a blocking layer or inversion layer that prevents the rise and dispersion of the warm air and the pollutants.

This is only likely to occur in low or no wind conditions.

Weather Stations and Inversion layers

Ground weather stations can measure the temperature at 2 heights (typically 1.5 metres and 10 metres) and this can be used to indicate the presence of an inversion layer.

Normally, the uppermost sensor will be at the same or a slightly lower temperature. But in an inversion condition, the 10 metre high temperature sensor will indicate a HIGHER temperature as this sensor is in the warm air BELOW the layer of cold air (the inversion layer).

In the atmosphere, the typical rate of cooling of 'dry' air (the 'Adiabatic lapse rate') is one degree per 100 metres. This difference would amount to about 0.1 degrees difference over the 10 metre height difference of the two temperature sensors. However, at the earth's surface, rates are typically much higher, which means the difference between the weather station temperature sensors at 1.5 and 10 metres is much more pronounced.

Other effects also occur near the surface, such as 'cool air drainage', where pockets of cold air tend to 'flow' downhill. These are called 'katabatic' winds, and are typically in the evening or early morning.

Sigma Theta

As still air is also a necessary condition for an inversion layer to occur, the variability of the wind direction can be used as an indicator of the amount of turbulence and therefore mixing of air. The term used is 'sigma theta', where sigma is a standard measure of variability and theta used to represent an angle.

Sigma Theta can be calculated from either the 'sum of sine and cosine' of the wind direction or the ratio of the vector (directional) wind speed to the scalar (non-directional) wind speed from the weather station.

Typical values for stable air are 0 - 3 degrees and for unstable air 20 - 30 degrees.

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7.Agrometeorological Indices

These indices are measure of environmental variability and are daily minimum temperature (Tmin), accumulated difference of maximum & minimum temperatures (TD), growing degree days (GDD), accumulated water vapour pressure deficit (VPD), sunshine hours (SH) & potential evapotranspiration (PET) etc.

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8. Weather & Climate

Weather is the condition of the atmosphere at a particular place over a short period of time. For example, on a particular day in a place, the weather is warm in the afternoon. But later in the day, when there are clouds blocking Sun's rays, the weather would become cooler.

Climate refers to the weather pattern of a place over a long period, maybe 25 to 30 years, long enough to yield meaningful averages . For example, although the weather in a place may be cool and dry today, that place's climate is hot today.

Elements of weather and climate

There are several elements that make up the weather and climate of a place. The major of these elements are five: temperature, pressure, wind, humidity, and rain. Analysis of these elements can provide the basis for forecasting weather. These same elements make also the basis of climatology study, of course, within a different time scale rather than it does in meteorology.

Elements of weather and climate :

1. Temperature is how hot or cold the atmosphere is, how many degrees it is above or below freezing (0°C). Temperature is a very important factor in determining the weather, because it influences or controls other elements of the weather, such as precipitation, humidity, clouds and atmospheric pressure.
2. Humidity is the amount of water vapor in the atmosphere.
3. Precipitation: is the product of any condensation process. It includes snow, hail, sleet, drizzle, fog, mist and rain.
4. Atmospheric pressure (or air pressure) is the weight of air resting on the earth's surface. Pressure is shown on a weather map, often called a synoptic map, with lines called isobars.
5. Wind is the movement of air masses.

Modifying factors of weather and climate

The more important are also five: latitude, altitude, distance to the ocean and/ or sea, orientation of mountain ranges toward prevailing winds and ocean currents.

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